Northern Ireland's finance minister Máirtín Ó Muilleoir is facing fresh calls to stand aside despite insisting he knew nothing about a secret back channel of communication linked to the biggest ever property deal in the North.

Mr Ó Muilleoir remained under mounting pressure last night as a Stormont committee prepared to meet to discuss the scandal.

Sinn Féin MLA Dáithí McKay resigned amid claims he and another party member coached a witness before he gave evidence to MLAs investigating the £1.3bn (€1.5bn) Nama sale.

Twitter messages were sent to loyalist blogger Jamie Bryson ahead of a committee meeting where he claimed Peter Robinson was to benefit financially from the deal - allegations the former First Minister strongly denies.

Mr McKay quit within hours of the messages being leaked, admitting his conduct was "inappropriate, ill-advised and wrong".

It has been claimed Mr McKay was sacrificed to protect more senior Sinn Féin figures including Mr Ó Muilleoir, a member of the finance committee at the time who is named in one of the messages.

Mr Ó Muilleoir said those seeking to link him to the controversy were indulging in "petty party politicking".

He said he had absolutely no knowledge of any communications between Mr McKay and Mr Bryson. "The attempts to link me to the contacts between Dáithí McKay, Thomas O'Hara and Jamie Bryson are no more than petty party politicking," he insisted. "They have absolutely no basis in truth or fact. I had no part in or knowledge of these inappropriate communications.

"I will co-operate readily and fully with any investigation."

But his refusal to stand aside pending an independent investigation sparked criticism.

Ulster Unionist leader Mike Nesbitt described it as a triumph of self-interest over the greater good, claiming it would damage public confidence in the devolved institutions.

The Nama sale has been dogged by months of controversy involving claim and counter-claim.

The 850-property portfolio was bought by Cerberus Capital Management in April 2014.

Last July, independent TD Mick Wallace claimed that £7m in an off-shore bank account linked to the deal had been earmarked for a Northern Ireland politician. Within days, Stormont's finance committee had begun an investigation.

Mr Bryson was one of a number of witnesses who gave evidence.

Messages show that on September 17 2015 - a week before Mr Bryson's appearance before the committee - Mr McKay sent him a direct message.

It told Mr Bryson to follow a Twitter account in the name of Mr O'Hara, a fellow Sinn Féin member.

In various messages he appears to advise Mr Bryson on how best to get his points across.

One said: "You may only get 10-15 seconds on this before Dáithí as chair has to pull you on it so squeeze your best points on this into 1-2 lines and come straight to the point."

Another refers directly to Mr Ó Muilleoir.

It states: "I'm trying to establish what Mairtin or someone could jump on and say there's no way we can turn him (Jamie Bryson) away, this is credible, relevant and in the public interest."